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Saab 99 Turbo part 12 – Learning To Use Auto Body Solder

I have seen body solder used in some custom car shows over the years, but have not tried it myself. It’s sometimes also called ”lead loading” since in the golden age of Hot Rodding lead was used to fill and shape body panels. Epoxy fillers (”bondo”) were not really available back in the day so body solder was used instead to even out the surface for paint.

Here’s an example:

Body solder used on 1959 Saab 93 roof and side panel seams.

People usually describe body solder use as difficult and a ”lost art”, so I kind of thought it as too time consuming to start learning it. But as a fellow Saab enthusiast decided to use it on his project car – a 1965 Saab Special – I decided to give it a go myself. Body solder does have some advantages over epoxy filler – it doesn’t attract moisture and it’s flexible. You can still work with a body panel after solder has been applied as it will form with the steel unlike bondo (it will crack).

But the more important thing is the fact that bondo will suck in moisture if it’s not completely sealed in. Danger spots are weld seams that may have minor holes or cracks in them where the water can come in from the back of the panel. This of course will ruin the paint some time later. Body solder on the other hand will fill the seam and keep the moisture from seeping under the paint.

So – how hard can it be? As it turned out – not hard at all.

Unleaded or leaded body solder?

There are basically two types of body solders. Lead free and leaded. There’s bit of a trade-off between these:

Unleaded is a lot safer but it is more difficult to use and requires more heat to melt. This in turn may cause the body panel that is worked on to warp from heat (happened to me on the rear quarter). It’s also a harder material which equals more work when sanding it down. And because of the high melting point it doesn’t stay in a malleable form so it’s near impossible to shape when applied.

Leaded body solder on the other hand contains lead, which is indeed poisonous. You definitely need to use glowes with it and preferably not breathe in any fumes. But it is a lot easier to use in my experience. It’s softer than the lead free solder so it’s faster to sand down. It is also relatively easy to keep in a buttery state by applying just the right amount of heat. This means you can shape, spread and smooth it out when working with it.

From left: Lead free and leaded body solder and on the right pure lead.

My recommendation: Try them both and see which you prefer.

In addition to the solder bars you need a small torch – a typical butane torch will do – and a wooden paddle to spread the solder around. You can soak the paddle in parafin or food oil to keep the solder from sticking to it.

Also soldering paste is needed for priming the surface. The paste is basically acid (flux) with a bonding agent and powdered solder. It also comes in leaded and lead free mixes. You also need some baking soda mixed with water to neutralize the acid after priming.

The process

Youtube is filled with videos on body solder usage – just search ”body solder”. Many guys there have a lot more experience than me so this is basically just for inspiration – even a complete beginner can get some pretty nice results.

The most difficult part is learning to control the heat. Use the torch with a small flame first. I used the lead free solder here as I only needed to fill a low spot and there was no need to shape the solder. Only to sand it down.

Here’s a great spot for body solder. The weld seam has pulled in a little because of the heat.

First order of business is to clean the area thoroughly. You can use a small sand blasting pistol and a sander. Sand blasted surface is not ideal for the primer so I prefer sanding over it.

Applying the solder paste. Just spread an even layer over the area.

The next step is to heat the paste to a point where it melts and then wipe it off with a clean cloth.

You should see a bright, even surface. The most common mistake is not heating the paste enough, so the solder in it doesn’t melt.

When the whole are has been wiped clean let it cool down. Then wipe it with a wet rag dipped in the baking soda and water mix. This will neutralize the acid and remove the solder paste remains.

Applying the lead free body solder is tricky. You can’t really shape it so you just basically melt it and let it sit where it is. Make sure you get enough heat in so it sticks to the primed surface.

Next order of business is to sand it down. Coarse files, sanding blocks or an orbital sander work fine.

Some sanding done but more is required.

An auto body file is a great tool. It’s a coarse file that is attached to a special holder. This one has a DIY holder my father made some time ago.

A body file reveals the straightness of the surface with a few strokes. Especially the softer body solder is pretty easy to file down. I prefer the orbital sander for the unleaded because it’s harder and takes longer to sand down.

Sanding the area pretty much done.

Looks pretty nice to me. The low spot is filled in with the body solder and it sealed what ever small holes might have been in the weld seam.

So – it’s not as difficult as some say. Now that I have some experience in using it I actually quite enjoy using body solder. And prefer to use it where I can. Of course epoxy filler still has it’s uses to fine tune the surface but from now on I will use body solder as much as possible.

A few more examples:

Here’ another example of weld seam shrinking where some filling is needed.

And after filling with body solder.

I also changed my plan on the rear quarter panel weld seams. I sand blasted and sanded them clean and used body solder to fill the gap.

Now that looks nice… I much prefer this approach to bondo.

And also a small low spot on the inner fender filled with body solder.